That's up to you. What size tank do you have? I didn't purchase the storage tank but I also didn't hook mine up under the sink. I have mine filtering water into a mixing tub. I use alot of water so the 100gpd works well for me.

I have a 58 and a 30. is the extra filtering worth it? the thing I dont understand is
it filters the tap water and then what? does it store it in the tank? does it know when to stop filtering when the tank is full? thanks for the help guys Im lost